Dave's Grinding Machine
for
Astronomical Objectives
Having made two mirrors by hand pushing I decided to make a machine to do the donkey work. It was a project that took about 3 years to complete which sounds a lot but it was done in bits and pieces around other jobs including the GEM, an observing platform for a 30inch Star Splitter DOB and various other household projects.
The biggest mirror I could see myself making was a 20 inch so everything was scaled accordingly. To ensure longevity all parts were made from galvanised steel, black zinc plated mild steel, aluminium, stainless steel or plastic of some kind.
Most parts are built up from smaller pieces of steel that were arc welded together and then machined. For example housing for the main shaft, shown in the first photo below, is built up from a piece of 3 inch galvanised pipe, the inside diameter of which was just smaller than the outside diameter of the intended bearing. This was welded to an offcut of 4 inch shafting that sawn off the bar many years ago and left outside in the weather to rust away which it nearly did judging be the depth of the rust pits. The two mounting bars were part of the packing case for a new 80 Tonne LVD Brake Press that the company I work for purchased in the year 2k. Once welded and machined to take the bearings, plated and assembled nobody could guess the heritage of each piece.
The main shaft and housing built around some 40mm and 38mm ID bearings that were excess to a 3 phase motor overhaul. I machined a 5 degree taper on the end of the shaft that makes fitting and removal of the mirror table easy and ensures the table runs true every time it is fitted.
I cut a large thread in the stroke shaft housing so I can adjust the height of the eccentric relitive to the mirror table. This is to adjust for different combinations of mirror and tool thicknesses.
The tangental gearbox rides on and can pivot around the main shaft and drives the 4 stepped pulley that drives the stroke shaft via a vee belt. The tangental nature of this box provides adjustment for the vee belt by the tensioned torque arm that is bolted to the gearbox. The gears are out of an old tapping machine and have 63 and 54 teeth and this odd ratio will ensure random tool strokes and the gearing reverses the direction of the stroke shaft with respect to the main shaft.
The machine is driven by an old 1/3 horse single phase motor via a 4 stepped pully and vee belt to the 50:1 gearbox that was once part of an oxy-acetylene profiling machine that was removed before the machine was scrapped. It resided under a friends house for 30 years before I got my hands on it. I stripped it down and found all the bearings, worm and wormwheel were in good condition. After kerosene wash for all parts, reassembly and a top up with new oil and finally a coat of paint she was as good as new. The output from the gearbox has considerable torque and to ensure safety to man, machine and optics I made a clutch consisting of a housing, clutch plate and pressure plate. The load on the two cork washers is applied by four M4 screws. I can now stall the machine by grabbing the mirror table which stops both shafts and the motor and 50:1 box carry on turning without faulter.
To precent undue loads on the bearings I installed a flexable coupling.
The frame is made from some galvanised 60x35x3 RHS welded together as shown and I certainly have no problem from flexure. Castors make moving the machine around the shed real easy.
A drip tray made from 3mm HDPE sheet with sides folded up at 45 degrees to both stiffen the tray and of course to keep the dripps from dripping over the working parts. Delrin was used for the flanges were turned up to seal the holes for the main shaft, stroke shaft and pivot shaft. The flanges were epoxied to the drip tray.
The mirror table is made from 16mm aluminium plate and made my little router grunt a bit but lots of little cuts made the going fairly easy.
I made a lexan housing to keep shed dust away from the grinding and polishing works and also to keep grits contained within and away from my lathe and other machinery. Access is gained via the lid that is hinged at the rear. An added bonus is the housing cuts down on the grinding noise and I can hear my radio while grinding with 80 grit, something that is difficult with the lid open.
Cleanup after a session is easy as the lexan housing, mirror table, stroke arms and drip tray can be taken outside and hosed down and washed on the grass, dried and reassembled ready for use.
My first mirror on the machine was a 10 inch f5 and I found it a delight to be able to get on with other projects while the machine worked away. This overhead shot shows the arangements of the hardware and I have indicated the movement and adjustments.
The knuckle movements between the two arms means the drive pin can be lifted off the tool and the arms raised to a verticle position, well clear of the mirror and tool, without having to disconnect anything.
The stroke is adjusted by a dovetail slide and leadscrew.
Despite some difficulty with a ceramic tile tool which was replaced with a glass tile tool, the 10 inch progressed nicely to a beautiful black polish. Final figuring was done by hand on my tri-legged table.
If you have any questions please send me an email as shown below, just remove the spaces.